The activity of the intrinsic exonuclease of Bacillus subtilis DNA polymerase III diminishes markedly with increasing chain length. Two opposing mechanisms account quantitatively for this effect: intrinsic competitive inhibition by interior substrate nucleotides, and increasing accessibility of the substrate terminus to the enzyme with increasing chain length. The generality of this explanation will be tested by performing a comparable analysis with other nucleases. Also, protein factors important in DNA replication will be tested for their suppression of intrinsic competitive inhibition. The properties of the nuclease of a mutator B. subtilis polymerase III enzyme will be examined to provide clues to the biochemical basis of its reduced fidelity in replication. Our preliminary data suggest a mechanism for the inhibition of DNA replication by cytosine arabinoside which will be tested rigorously by genetic and enzymological procedures. The physiological role of phage T4 RNA ligase and its use as a synthetic reagent for DNA synthesis will be examined further. BIBLIOGRAPHIC REFERENCES: Cozzarelli, N.R., Low, R.L., Rashbaum, S.A., and Peebles, C.L. (1975) DNA polymerase III from Bacillus subtilis and the mechanism of arylhydrazinopyrimidine inhibition. In: DNA Synthesis and its Regulation, (Goulian, M., Hanawalt, P., and Fox, C.F., eds), pp. 14-37. Low, R.L., Peebles, C.L., Rashbaum, S.A., and Cozzarelli, N.R. (1976) Bacillus subtilis DNA polymerase III from wild-type and mutant cells. Microbiology - 1976. In Press.